In the art of security printing, security documents, such as banknotes, typically involve a so-called intaglio printing operation making use of intaglio printing plates (or like printing mediums) having complex curvilinear patterns engraved in the surface of the intaglio printing plates (see e.g. International Publications Nos. WO 03/103962 A1, WO 2007/119203 A1 and WO 2009/138901 A1 which are all incorporated herein by reference).
Nowadays, intaglio printing plates for the production of security documents are typically produced by laser engraving of a polymer precursor plate (see e.g. WO 03/103962 A1) or by direct laser engraving of a metallic plate (see e.g. WO 03/103962 A1 and WO 2009/138901 A1). Alternative solutions include the direct mechanical engraving of a metallic plate using rotating chisels or the like, or etching of a metallic plate which has first been provided with a suitable etch-resistant mask by photolithographic techniques. In all cases, prior to being used on an intaglio printing press, the intaglio printing plates are first provided with a wear-resistant coating of, usually, chromium with a view to increase the surface hardness of the intaglio printing plate, and therefore its resistance to wear, as well as the resistance to corrosion.
To date, intaglio printing plates are commonly chromed by electroplating (or “chrome plating”), i.e. by subjecting the uncoated intaglio printing plates to galvanic processing in a chromium bath. Chrome plating is however potentially problematic from a health standpoint due to the production of hexavalent chromium (or “chromium VI”) compounds which require suitable treatment and careful handling.
A potential alternative to chrome plating is the deposition of chromium (or similar wear-resistant coatings) by physical vapour deposition (PVD) techniques. PVD is a process whereby atoms are ejected from a solid target material usually by ion or atom bombardment in a vacuum environment and then deposited onto a desired substrate.
Deposition of wear-resistant coating materials onto printing mediums, especially engraved printing mediums, is already known as such in the art. U.S. Pat. No. 5,252,360 (and corresponding European Patent No. EP 0 446 762 B1) for instance discloses a process for the protection of an engraved roll or plate, for example for gravure printing, wherein a base body of metal whose surface is provided with engraved cells is coated with at least one layer of a metal or metal compound in order to increase the wear resistance and the corrosion resistance of the engraved roll or plate. According to U.S. Pat. No. 5,252,360, a dense interlayer of a metal-containing component or ceramic-containing component having a thickness of about 10 to 15 microns and a Vickers hardness of at least 850 HV is first provided onto the surface of the engraved roll or plate. This interlayer is for instance applied by spraying or electrolytically. The surface of this interlayer is thereafter polished and cleaned, and then subjected to vacuum. While being kept under vacuum, the base body is heated up for tempering purposes to a temperature of at least 240° C. to about 480° C. for a time period of at least 1 hour up to abut 4 hours. Then, at the end of the tempering period, while the tempered engraved base body with its interlayer is continuously subjected to vacuum and kept in a heated up condition between 200° C. and 480° C., a wear-resistant layer of a metal compound having a Vickers hardness of at least 2000 HV is formed by physical vapour deposition in a thickness of about 4 to 8 microns, this vapour-deposited wear-resistant layer being ultimately polished.
U.S. Pat. No. 5,252,360 does not specifically disclose or refer to any particular type of coating apparatus to perform PVD deposition of the wear-resistant material. German Patent Publication No. DE 195 16 883 A1 similarly discloses an engraved printing medium for gravure printing, which engraved printing medium is provided with a thin layer of wear-resistant coating material. DE 195 16 883 A1 contains a vague reference to the use of a vacuum coating apparatus for performing the deposition of the wear-resistant coating material, but does not precisely describe the relevant vacuum coating apparatus.
PVD coating apparatuses are known in the art for applications other than for the coating of printing plates. These apparatuses are however not directly suitable for the processing of intaglio printing plates for the production of security documents. The known PVD coating apparatuses are typically either too small or too large and unable to deposit coatings of the desired properties and thicknesses as required in the context of the production of intaglio printing plates. Such is the case for instance of the PVD coating apparatus disclosed in U.S. Pat. No. 4,892,451 which specifically relates to the processing of data storage disks, such as CDs and magnetic disks, semiconductor wafers, and like substrates in plate form for optical and/or electronic purposes.
Similarly, the coating method disclosed in US Patent Publication No. US 2011/0139246 A1 is specifically applied for the deposition of a thin transparent conductive oxide (TCO) layer on a substrate for use in photovoltaic devices, which method is not directly transposable to the coating of intaglio printing plates for the production of security documents.
An improved solution is thus required.